1. Field of the Invention
This invention relates to a thin-film piezoelectric element that can be applied to ink-jet printers, scanners, gyros, ultrasonic generators, ultrasonic sensors, pressure sensors, and velocity/acceleration sensors and, particularly, to a thin-film piezoelectric element using lead-free piezoelectric material and having excellent dielectric strength voltage.
2. Description of the Related Art
Piezoelectric substances are processed into various piezoelectric elements according to various purposes, and are utilized widely particularly as functional electronic components, such as an actuator, which applies voltage to cause deformation, a sensor, which conversely uses deformation of an element to generate voltage.
As piezoelectric substances utilized in actuator and sensor applications, there have hitherto been widely used lead-based ferroelectrics having large piezoelectric properties, especially, Pb(Zr1—xTix)O3-based perovskite-type ferroelectrics called PZT, which are formed by sintering an oxide typically comprising individual elements.
At present, with reduction in size and enhancement in performance of electronic components of each kind, there is also a strong demand for size reduction and performance enhancement of piezoelectric elements as well. However, in piezoelectric material produced by a manufacturing method using primarily a conventional sintering method, as its thickness is made thin, particularly, as the thickness approaches a thickness of the order of 10 μm, and a size of crystal grains constituting the material, the effect of the thickness becomes unnegligible.
For that reason, there arises the problem of noticeable property variation and degradation. In order to avoid it, the formation methods of piezoelectric substances have, in recent years, been studied that apply thin film technology, etc. replaced for the sintering method.
As the thin film technology replaced for the sintering method, a PZT thin film formed by RF (high-frequency) magnetron sputtering has recently been used practically as an actuator for heads of high-definition high-speed inkjet printers (See JP-A-2004-260158, for example).
On the other hand, sintered piezoelectric substances or piezoelectric thin films comprising above-mentioned PZT are undesirable from the points of view of ecology and pollution prevention because they contain lead oxide (PbO) on the order of 60-70 wt %. Accordingly, from environmental concerns, the development of piezoelectric substances that do not contain lead is desired.
At present, various lead-free piezoelectric materials have been studied, of which there is lithium potassium sodium niobate (general formula: (NaxKyLiz)NbO3 (0<x<1, 0<y<1, 0≦z<1, x+y+z=1) (See JP-A-2004-300012, for example). A thin film of this lithium potassium sodium niobate is a lead-free material having perovskite structure that exhibits a relatively good piezoelectric property, and is therefore expected as a major candidate for lead-free piezoelectric material.
However, there is the problem that the thin film of lithium potassium sodium niobate has very low dielectric strength voltage. In particular, because, in the case of use in the form of a piezoelectric thin film, the electric field strength to be applied to the piezoelectric thin film becomes large, this low dielectric strength voltage is a serious problem that significantly obstructs practical use.